Corroded battery terminals

The battery posts and the clamps on them have corroded, so the connection carries current badly even though it looks connected.

The characteristic behaviour is that voltage falls in proportion to demand, because resistance and current produce a voltage drop between them. At rest everything measures fine; the moment something draws heavily the voltage at the far end sags. So the symptoms arrive in company and look unrelated to each other — a module resetting during cranking, several warning lights appearing together, a reading that goes out of range only under load. A spread of odd electrical faults with no common component is very often one bad joint rather than several coincidental failures.

The white or blue-green crust is a symptom rather than the whole fault. It is a product of acid vapour escaping around the post, and the part that matters is the film between clamp and post underneath it, which is thin, invisible and where the resistance actually lives. A terminal that has been wiped clean on the outside can still be a bad connection, so the joint is undone, both faces cleaned, and the clamp tightened properly rather than polished in place.

Two things make it worth ruling out early on almost any electrical complaint. It is common, and it is close to free — which together mean it should be eliminated before anything is diagnosed, not after. On a vehicle where a battery has recently been changed, add a third: a clamp that was never fully tightened behaves exactly like a corroded one, and the fault dates from the work rather than from the weather.